An oscillator with an adjustable ("pullable") output frequency is useful in many applications. In a conventional crystal-based oscillator, the output frequency is a function of the load capacitance seen by the crystal. To make the output frequency adjustable, therefore, the load capacitance can be made variable. A variable capacitance is often provided by a varicap, i.e., a diode which is designed to provide a wide range of junction capacitance within a narrow range of reverse voltages. The reverse voltage controls the frequency and is often generated by a digital-to-analog converter.
One problem with this technique is that the varicap is not available in digital processes. Consequently, the varicap and supporting circuitry are provided externally. To build this type of oscillator using only one varicap requires a type of circuit that is not easily integrated in a digital Complementary Metal-Oxide-Semiconductor (CMOS) process, because it requires floating capacitors (i.e., capacitors which have neither end coupled to Vcc or Vss).
Another problem associated with certain prior art oscillators is that they tend to waste power. To ensure proper start-up under all conditions, many prior art oscillators are designed to have excess gain, beyond what is needed to start and maintain oscillation in most situations. As a result, the oscillation amplitude increases until limited by non-linearities and usually exceeds the supply voltage. This level of drive wastes power, which can be particularly problematic in battery-powered applications. Further, this approach stresses the crystal. This approach also complicates integration of the required load capacitances in a digital CMOS process, due to the wide voltage swing across the capacitors. Consequently, external capacitors are normally provided.